These findings indicate that depression and anxiety are each associated with multiple domains of health-related quality of life in cancer patients. Group comparisons suggested that participants with no depression or anxiety had better HRQL than did those with depression only, and that those with comorbid depression-anxiety reported worse HRQL than did the depression-only group. Moreover, the participants in the depression and anxiety groups also self-reported incrementally more disability on both an overall measure (Sheehan Disability Index) and a measure of the number of disability days. In interpreting these findings, it is important to note that the 88 patients in the reference group with no depression or anxiety did meet criteria for pain, making it an atypical comparison group. However, since pain itself causes pervasive disability across multiple domains of HRQL and functional status (29
), it is likely that even greater differences would be found if we had compared a depressed group and a group with depression and anxiety to a group of cancer patients with no pain as well as no anxiety or depression. Thus, our findings of the relationship between depression/anxiety and HRQL may be conservative given the nature of our reference group.
GLM regression modeling further supported significant linear associations of depression severity and anxiety severity on HRQL and disability, all in the expected direction. Associations were significant for each symptom across all domains when modeled separately. The findings are more equivocal, however, about the degree to which anxiety's relationship to these outcomes exists independent of depression. When the two variables were entered into the models together, depression showed a significant association across all domains, but anxiety associations were not found to be significant in a majority of the domains after depression had been accounted for.
Of the three domains in which significant independent associations of anxiety were
found, two were mental health constructs—SF Mental Health and the SF Mental Component Summary. It is not surprising that anxiety would show an association independent of depression in these domains, which are intended to represent psychological symptoms. Somatic symptom severity was the other variable in which anxiety showed an association over and above that of depression. This finding is consistent with previous evidence in noncancer medical populations of the strong associations among anxiety, depression, and somatization – the “SAD triad” (30
). The current finding does contradict, however, a recent finding that neither anxiety nor depressed mood was associated with physical symptoms in a sample of hospitalized advanced cancer patients (1
). The explanation for the incongruent findings may be difference in measurement and/or differences in the cancer patient sample, since the current data is derived from a more representative sample of patients with different types and stages of cancer .
Our results suggest that depression has more pervasive effects than anxiety on non-mental domains of quality of life in cancer patients. Anxiety had an effect distinguishable from depression on mental health and functioning as well as somatic symptom severity. Perhaps these findings could be considered consistent with the tripartite model of anxiety and depression (10
). This theory holds that anxiety and depression represent two affective syndromes with some components in common and some that are unique. According to the tripartite model, anxiety and depression share a general distress factor. An additional structural component that is specific to anxiety, however, is physiological hyper-arousal, whereas a component over and above general distress that distinguishes depression is anhedonia. It could be that accounting for depression in our GLM analyses represented a partialing out of the general distress that is common to both anxiety and depression. Factors specific to anxiety, then, might explain why the comorbid anxiety-depression group fared worse than the depression-only group on most domains in our group comparisons. It might also explain the independent association of anxiety over and above depression on the mental health domains in this study. In the context of the tripartite model, our findings may merely suggest that in cancer patients, the general distress component is proportionally more influential than the unique factors of anxiety and depression on domains of HRQL. INCPAD was by design oversampled for depression, which may have contributed to the more pervasive findings for depression. An important next step would be to conduct similar analyses in a sample with a relatively even proportion of patients with anxiety, and including an anxiety-only group in the comparisons.
A limitation to this study is its cross-sectional design, which precludes any interpretations about causality or directionality. While it is tempting to conclude that anxiety and depression have negative effects on HRQL, it is also possible that lower HRQL contributes to severity of anxiety and depression or that a cancer diagnosis or other cause has simultaneous effects on HRQL as well as depression and anxiety. Moreover, it is possible that the lower self-reported HRQL might be related to the tendency of a depressed person to make more negative appraisals of HRQL. Other important limitations have already been alluded to—the oversampling of depression relative to anxiety and the fact that the reference group is a clinical sample with pain symptoms.
In summary, depression and anxiety are independently associated with mental health domains and somatic symptom severity in a heterogeneous sample of cancer patients who screened positive for depression or pain or both. In other important domains of HRQL and disability (vitality, general health, overall quality of life, and disability), anxiety and depression had significant associations when analyzed separately; however, when entered into a general linear model together, anxiety did not show an independent association after depression had been accounted for. The overall findings of this study are consistent with a large body of literature that suggests that, although the overlap of the two symptoms is pronounced, anxiety is distinguishable from depression in some—although not all—contexts (9
). It also highlights the importance of recognizing and treating depression and anxiety in oncology practice, not only for the suffering caused by the disorders themselves but also for their adverse effects on multiple domains of HRQL.